A fluororesin is excellent in the solvent resistance, low dielectric properties, a low surface energy, non-adhesiveness and the weather resistance, and accordingly used for various applications for which a general purpose plastic cannot be used. Particularly, an ethylene/tetrafluoroethylene copolymer (hereinafter tetrafluoroethylene will sometimes be referred to as TFE, and an ethylene/tetrafluoroethylene copolymer as ETFE) is a fluororesin excellent in the heat resistance, the flame retardancy, the chemical resistance, the weather resistance, the low friction property, low dielectric properties, the transparency, etc. and is thereby used in a wide range of fields, such as a covering material for a heat resistant electric wire, a corrosion-resistant piping material for a chemical plant, an agricultural plastic greenhouse material, and a mold release film.
However, unlike polyvinylidene fluoride which is soluble in N-methylpyrrolidone and the like, ETFE is usually insoluble in a solvent and cannot be applied to formation of a thin film e.g. by coating, and accordingly its forming method is limited to melt forming such as extrusion, injection molding or powder coating.
Heretofore, an attempt to obtain a solution of ETFE has been reported. An ETFE solution was obtained by using as a solvent a dicarboxylic diester such as diisobutyl adipate, however, the dissolution temperature must be a high temperature of 230° C., 260 to 265° C. or 290° C. (Patent Documents 1 to 3). Further, an example of using a low molecular weight chlorotrifluoroethylene polymer as a solvent has also been reported, but heating to a temperature in the vicinity of the melting point of the polymer is still required. Further, it is disclosed that the solution can hardly be used for coating since the boiling point of the solvent is high, and that a dispersion liquid obtainable from the solvent loses fluidity in the vicinity of room temperature (Patent Document 4). On the other hand, an example has been reported that ETFE is utilized for flash spinning using a ketone, a hydrofluorocarbon or the like as a solvent under high temperature and high pressure conditions. However, any attempt is carried out under very high pressure conditions of at least 13 MPa, and accordingly a special apparatus is required, and application other than the aimed purpose is difficult, and for example, it is impossible to apply such an attempt to production of a thin film, a film, a porous material such as a hollow fiber (Patent Document 5).
Accordingly, examples in the past are hardly easily conductible in practical operation, and an easily handleable technique or method to obtain a solution of ETFE at a relatively low temperature has not been known.
Patent Document 1: U.S. Pat. No. 2,412,960
Patent Document 2: U.S. Pat. No. 2,448,952
Patent Document 3: U.S. Pat. No. 2,484,483
Patent Document 4: U.S. Pat. No. 4,933,388
Patent Document 5: JP-A-2000-503731